1. Field of the Invention
This invention relates to laminated printing plates which are photoengraved for use in printing articles. More particularly, this invention relates to laminated printing plates comprising a metal such as magnesium and zinc which are bonded to a self-supporting, dimensionally stable base structure. This invention also relates to a method for producing laminated printing plates.
2. Description of the Prior Art
The process of photoengraving is widely used in virtually all forms of printing for printing newspapers, books, packaging and containers of all sorts. Basically, conventional photoengraving process involves coating a metal plate, such as magnesium, zinc and their alloys, with a photoresist layer. The photoresist layer is exposed to a desired negative containing the printing to be performed. After the exposed photoresist layer is developed, the metal plate is placed in an aqueous, nitric acid bath which etches all of the unprotected surface areas of the metal plate. When the desired relief is obtained, the metal plate is removed from the bath and washed. In many applications, the metal plate is bent to a curved design and fitted onto the outer surface of a print cylinder of a printing machine. Means are usually provided for properly registering the metal plate on the cylinder.
Presently there exist many types of metal plates of various thicknesses which are used in the photoengraving process. Some of these metal plates are plated or laminated with other materials including other metals and plastics. For example, U.S. Pat. No. 2,704,512 discloses a magnesium base metal printing plate onto which is plated a skin of zinc of uniform thickness in the order of 0.003 inch to 0.0003 inch. This plate was designed to be etched by the powder etching method common during that period of time. However, since the development of powderless etching techniques, this plate has become obsolete. U.S. Pat. No. 4,119,035 discloses a flexible metal printing plate comprising a layer of copper bonded to the top surface of the base layer of iron with a layer of chromium bonded to the top surface of the copper and a layer of zinc bonded to the undersurface of the iron layer. This plate, as a wet offset plate, would be unsuitable as a dry offset plate in many respects. U.S. Pat. No. 3,361,063 discloses a printing plate comprising an etchable metal bonded to a metal backing layer by means of an adhesive layer encapsulating at least one layer of woven glass cloth. The glass cloth allows the combined etchable layer and metal backing layer to be compressed with great force without squeezing out all of the adhesive since the glass cloth functions as a stop, determining the thickness of the adhesive. However, the use of glass cloth produces a weak bond at all of the points which the cloth strands contact the compressed layers. A weak bond is unacceptable for many applications. For example, process color applications involve reproducing pictures by using very fine "dots" etched on the printing surface of the printing plate. Those dots which are located over a strand of the cloth are not sufficiently adhered to the plate, and will, during use, lose their bond with the plate and break away, resulting in inferior printing. U.S. Pat. No. 3,929,073 discloses a printing plate comprising an aluminum-zinc cladding plate which is anealed to an aluminum alloy plate. This plate is not usable on a magnetic cylinder, is not strong enough to clamp to a cylinder without stretching, and is extremely difficult to etch. Finally, U.S. Pat. No. 4,136,615 discloses a laminated plate in which a sheet of magnesium is laminated to a self-supporting, dimensionally stable, all plastic base structure. While easier to etch, this plate suffers from the same disadvantages of the latter plate.
Each of these printing plates disclosed in the above referenced patents, the disclosures of which are hereby incorporated by reference herein, have contributed in some form or another to the advancement of the art. However, there are numerous disadvantages to these printing plates, some of which were outlined above.
Therefore, it is an object of this invention to provide an apparatus and a method which overcomes the aforementioned inadequacies of the prior art devices and provides an improvement which is a significant contribution to the advancement of the printing plate art.
Another object of this invention is to provide a laminated printing plate composed of a sheet of magnesium, or other etchable metal, bonded to a metal layer by means of an adhesive.
Another object of this invention is to provide a laminated printing plate having a thickness which is compatible with print cylinders of existing rotary printing machines.
Another object of this invention is to provide a laminated printing plate which is not adversely affected by etching acids and other chemicals commonly used in the photoengraving processes.
Another object of this invention is to provide a laminated printing plate which can be used in long runs and at high speeds in the printing of articles without degradation of the printing plate itself or the quality of the printing.
Another object of this invention is to provide a laminated printing plate which may be curved to the radius of the print cylinders of rotary printing machines without unwanted distortion of the photoengraved print in the magnesium.
Another object of this invention is to provide a laminated printing plate composed of a sheet of etchable magnesium which, upon curving to fit a print cylinder, produces a matte finish to facilitate the transfer of ink from the ink roller to the article being printed in a more uniform manner than that which could be accomplished by prior art printing plates.
Another object of this invention is to provide a laminated printing plate composed of stock materials of conventional sizes which are available from multiple sources.
Another object of this invention is to provide a method for producing a laminated printing plate in which a sheet of magnesium, or other etchable metal, is bonded to a metal layer by means of an adhesive.
Another object of this invention is to provide a method for producing a laminated printing plate in which the various materials used throughout the method can be purchased as stock items from multiple sources.
Another object of this invention is to provide a method for producing a laminated printing plate which can be accomplished by machines and equipment normally found in complete photoengraving businesses, thereby eliminating the need for purchasing expensive tooling and equipment.
Another object of this invention is to provide a method for producing a laminated printing plate which can be accomplished on a relatively small scale without a large capital investment over and above the equipment normally found in complete photoengraving businesses.
The foregoing has outlined some of the more pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.